With constant evolution of wireless communication technologies and standards, mobile packet services have been developed tremendously, and data throughput capacity of a single terminal is constantly upgrading. Take the Long Term Evolution (referred to as LTE) system as an example, a maximum downlink data transmission rate of 100 Mbps is supported in 20 M bandwidth, and in the subsequent LTE Advanced network, the data transmission rate will be further increased, even up to 1 Gbps.
The related LTE user plane data protocol stack is shown in FIG. 1, the downlink data received by an evolved nodeB (Evolved eNB) from the core network through the GPRS Tunneling Protocol for the user plane (referred to as GTP-U) is unpackaged and then sent to the User Equipment (abbreviated to UE) after processed by the packet data convergence protocol (referred to as PDCP) sub-layer, the Radio Link Control (referred to as RLC) protocol sub-layer, the media access control (MAC) protocol sub-layer and the physical (PHY) layer; the uplink data transmission is exactly opposite to the downlink one.
Currently, the data transmission link between the network side and the UE is a one-to-one dedicated link, therefore the signal quality and the size of the used resource of this link determines the data transmission performance between the network side and the UE. If the resource used by the link is restricted or the signal quality is relatively poor, the user experience of the UE will fall down, which is a great challenge now faced by mobile operators, although the network capacity extends year by year, it still cannot keep up with the increasing number of user terminals and the increasing user demand on data traffic.
In order to meet the growing demand on data traffic as well as the geographically uneven characteristics of the traffic, the operators add Low Power Nodes (LPNs), Small Cells or Pico eNBs to increase hotspots in the process of deploying the new generation of communication network (such as the LTE). With the increasing number of LPN cells, the network deployment environment becomes more complex, meanwhile it also brings some problems.
First, because the coverage of an LPN cell is much smaller compared to a Macro Cell, the capacity is relatively small, some LPN cells could easily be occupied by users, leading to the high load, thus affecting the user data throughput, while some other LPN cells or macro cells are at a relatively low level of load. In order to balance the load, the network side needs to perform load balancing operation, but this process is not flexible enough, especially when the number of cells is relatively large, this uneven load becomes more serious because of a lack of flexibility.
In addition, because the number of LPN cells is relatively large, when the user equipment, also called user terminal, moves within the network, it will lead to frequent inter-cell handovers, and causes frequent data service interruption or even call dropped, which causes user data throughput and user experience fall down. At the same time, this frequent handover results in the terminal and the network, especially the core network, receiving an impact of a large number of signaling, which may lead to a congestion and even paralysis of system resources.
With the increasing number of LPN cells deployed by operators and individuals in the future, the abovementioned situation becomes increasingly serious, therefore nowadays many companies and operators are inclined to looking for new enhancing schemes, and Dual Connectivity is one of them. Terminals in the dual connectivity can simultaneously remain connected with two network nodes (or more than two, the dual connectivity as described herein is just a general term and does not limit the number of connections), shown in FIG. 2, wherein the master node is called master eNB (MeNB, generally refer to a macro evolved NodeB node) or master base station, while other nodes are called Secondary eNB (SeNB, generally refer to micro evolved NodeB or low-power node) or secondary base station, and for example, the UE remains connected with the macro cell and the LPN cell at the same time, when the network load is not balanced, the network side can adjust the amount of data transmitted by the terminal in the MeNB and SeNB nodes in real time, and at the same time, if the SeNB cell changes because the UE moves or due to other reasons, the other cell can still stay connected, and this change will not lead to excessive signaling impacts.
There are many traffic offloading methods between the MeNB and the SeNB, the offloading anchor point may be placed in the serving gateway (S-GW), as shown in FIG. 3 (A); or may be placed in the MeNB, if it is placed in the MeNB, it may also continue refining the traffic offloading collaboration between different layers depending on their different specific traffic offloading layers, such as traffic offloading in the PDCP layer, traffic offloading in the PDCP layer, etc., as shown in FIG. 3 (B), FIG. 3 (C), FIG. 3 (D), and FIG. 3 (E).
The abovementioned dual connection method has been enhanced for frequent changes of the SeNB, but when the MeNB changes, the migration of all UE related context, including part of the configuration information carried in the SeNB, cannot be achieved in the dual connection handover in accordance with conventional handover methods. The MeNB has to first get back the connection bearer in the SeNB, convert it to a single connection, and then the new MeNB reselects a SeNB to offload the traffic after the MeNB handover completes. This will increase the probability of issues such as data traffic interruptions and dropped calls, therefore, it still needs to be further optimized.